The present invention relates to methods and compositions for treating subterranean well formations, and more specifically, to improved orthoester compositions comprising orthoesters and methods for reducing the viscosity of viscosified treatment fluids.
A variety of viscosified treatment fluids are used in subterranean applications, such as drilling fluids, fracturing fluids, and gravel pack fluids. Oftentimes, after the viscosified fluid has performed its desired task, it may be desirable to reduce its viscosity so that the treatment fluid can be recovered from the formation and/or particulate matter may be dropped out of the treatment fluid at a desired location within the formation. Reducing the viscosity of a viscosified treatment fluid is often referred to as “breaking” the fluid.
Well stimulation treatments, such as fracturing treatments, commonly employ viscosified treatment fluids. Fracturing generally involves pumping a viscous fracturing fluid into a subterranean formation with sufficient hydraulic pressure to create one or more cracks or “fractures.” The fracturing fluid generally has a viscosity that is sufficient to suspend proppant particles and to place the proppant particles in fractures, inter alia, to maintain the integrity of those fractures once the hydraulic pressure is released. Once at least one fracture is created and the proppant is substantially in place, the viscosity of the fracturing fluid usually is reduced, and the fluid is recovered from the formation.
Similarly, sand control operations, such as gravel packing, use viscosified treatment fluids, often referred to as gravel pack fluids. Gravel pack fluids usually are used to suspend gravel particles for delivery to a desired area in a well bore, e.g., near unconsolidated or weakly consolidated formation particulates. One common type of gravel packing operation involves placing a gravel pack screen in the well bore and packing the annulus between the screen and the well bore with gravel of a specific size designed to prevent the passage of formation sand. When installing the gravel pack, oftentimes the gravel is carried to the formation in the form of a slurry by mixing the gravel with a transport fluid. The gravel, inter alia, acts to prevent the particulates from occluding the screen or migrating with the produced fluids, and the screen, inter alia, acts to prevent the gravel from entering the production tubing. Once the gravel pack is substantially in place, the viscosity of the gravel pack fluid often is reduced to allow it to be recovered from the well bore.
For some viscosified treatment fluids their viscosity may be related to pH. Thus, viscosity-reducing agents that reduce the pH of the treatment fluid may be added to reduce the viscosity of the fluid. Internal breakers, such as enzymes, oxidizers, acids, or temperature-activated viscosity reducers, also are used to reduce the viscosity of viscosified treatment fluids. Unfortunately, these traditional breakers may result in an incomplete or premature viscosity reduction. Premature viscosity reduction is undesirable as it may lead to, inter alia, the particulates settling out of the fluid in an undesirable location and/or at an undesirable time. Moreover, conventional non-delayed breakers begin to reduce the viscosity of the viscosified fluid upon addition and continue to reduce the fluid's viscosity with time until the fluid is completely broken or until the breaker is expended. Since the breaking activity begins immediately, it is common practice to start with excess viscosifier to offset the point at which the viscosity falls below an acceptable level. Using excess viscosifier is not only an added expense, it also may lead to excessive friction pressure during treatment placement.
As an alternative to using traditional breakers, breaking a viscosified treatment fluid also may be accomplished using just time and/or temperature. The viscosity of most treatment fluids will reduce naturally if given enough time and at a sufficient temperature. However, such methods generally are not practical as it is highly desirable to return the well back to production as quickly as possible as opposed to waiting for the viscosity of a treatment fluid to naturally decrease over time.